1. Field of the Invention
The invention relates generally to capacitance humidity sensors, particularly to humidity sensors having a moisture sensitive dielectric layer interposed between a pair of conductors, and still more particularly to the use of a certain class of polyimide resins as the dielectric layer, i.e. those having at least one ether linkage in its dianhydride portion.
2. Description of the Prior Art
In commonly-owned U.S. Pat. No. 4,965,698 issued Oct. 23, 1990 to Thoma, et al. for "Capacitance Humidity Sensor", hereafter the '698 patent, a device of the general type with which the resin of the present invention is particularly well suited is described in considerable detail. The device of the '698 patent includes a core made of a material whose dielectric constant varies substantially linearly with humidity, such as a polyimide or polyparabanic acid material. The core is sandwiched between layers of polymeric material having conductive particles, such as conductive carbon particles, dispersed therein. In the '698 patent, the resin matrix for the conductive layers is described as containing certain crosslinked polymers formed by the reaction of a compound containing glucoside chains and a monomer or partial polymer capable of reacting with the hydroxyl groups of the glucosides. Specific examples of the disclosed materials included cellulose nitrate, cellulose triacetate, cellulose butyrate and, most preferably, cellulose acetate butyrate (CAB) crosslinked with urea formaldehyde or melamine formaldehyde.
Reference is made in the background section of the '698 patent to other types of known humidity sensors which include a dielectric layer and thin metal electrode conducting layers. Reference should be made to the patents cited in column 1 of the '698 patent for examples of such systems. The prior systems discussed in the earlier patent were deemed to have poor corrosion resistance, since thin metal electrodes can rapidly be destroyed by sulfur-based pollutants or chlorine.
Polyimides have been recognized as a particularly useful dielectric for humidity sensors because their dielectric constant is substantially linearly proportional to the moisture content. However, bonding between the polyimide and prior art metal foil or electrically conductive particle electrode layers was difficult without the use of adhesives because of the dissimilarity between the metal or electrically conductive particles and plastic. Examples of such prior art systems include U.S. Pat. Nos. 3,350,941 issued to Misevich in Nov. 1967; 4,429,343 issued to Freud on Jan. 31, 1984; 5,050,434 issued to Demisch on Sep. 24, 1991; and 5,069,069 issued to Miyagishi, et al on Dec. 3, 1991.
The '698 patent also discusses the use of mixtures of conductive particles, such as particles of silver or carbon black, dispersed in resins such as polyimides as shown, for example, in the Takenaka, U.S. Pat. No. 3,697,450 issued Oct. 10, 1972. It is also pointed out that other humidity sensors have employed various layers of crosslinked resins such as CAB crosslinked with urea formaldehyde. In one sensor, a crosslinked cellulose acetate butyrate core, containing conductive particles such as carbon, is sandwiched between a pair of outer resin layers free of carbon particles. See Thoma, U.S. Pat. No. 3,458,845 issued Jul. 29, 1969. In other humidity sensors, the outer resin layers contain the conductive particles and the inner resin layer does not; see Thoma, U.S. Pat. Nos. 3,582,728 issued Jun. 1, 1971; 3,802,268 issued Apr. 9, 1974; and IEEE Transactions On Components, Hybrids and Manufacturing Technology, Vol CHMT-2, No. 3, 1979, pgs. 321-323. Baxter, et al. U.S. Pat. No. 4,564,882 issued Jan. 14, 1986 describes a humidity sensing element wherein the dielectric layer can be made from either CAB or polyimide. Reference should also be made to the prior Thoma, et al. patent for discussions on the background use of polyparabanic acid polymers and on screen printing technologies.
In another commonly owned patent, U.S. Pat. No. 5,177,662 issued to Thoma on Jan. 5, 1993 for "Capacitance Humidity Sensor", the conductive layers utilize sulfur atoms in the backbone chain of the resin with conductive particles dispersed therein to improve corrosion resistance and performance.
The present invention addresses certain remaining drawbacks with such capacitance humidity sensors. Specifically, it is desirable to improve overall performance with a resin providing highly desirable humidity sensing characteristics and water vapor transmission rates and water absorption.